The field of the invention is that of the power supply to electronical circuits, and in particular DCDC circuits (DC voltage converters), designed to supply a load current with an output voltage that is different from the input voltage. More precisely, the invention concerns the detection of possible overloads, in order to prevent the powered circuit from being damaged.
The invention has applications in many different fields, wherever a voltage needs to be modified and a supply current checked. One particular example of an application is that of the power supply to a chip card (SIM card, credit card, etc.) co-operating with a radio telephone, supplying a variable voltage from its battery.
There are several types of DCDC converters known. In particular, there are switched capacity converters, that provide unsatisfactory performances and inductive converters, which are more especially concerned by the invention.
The principle of an inductive converter is based on the fact that an inductance opposes the current variations, by increasing or decreasing the voltage at its terminal according to the rule: dV/dt=L.di/dt.
The purpose of the converter is to increase (pump mode) or to decrease (regulation mode) an output voltage (Vout) with respect to a supply voltage (Vbat). For example, in the case of a chip card powered by a radio telephone, the output voltage should be 5 V permanently, whereas the battery can supply a voltage of between 2.7 and 5.5 V according to its charge.
FIG. 1 illustrates the operation of an inductive converter in pump mode, which is to say when Vbat>Vout. It therefore has an inductance L, of a value of 4.7 μH for example, one end of which is connected to the battery (Vbat) and the other end (IN) is connected on one side to an NMOS TN0 transistor and on the other side to a PMOS TP0 transistor.
The latter transistor provides the regulated output voltage Vout acting on the OUT output. Regulation devices send a command H, to the two transistors TN0 and TP0. The TN0 transistor is conductive when H=Vbat and blocked when H=0V, contrary to TP0 which is conductive when H=0 V and blocked when H=Vbat. They are therefore complementary.
This set-up permits a current to be imposed in the inductance L for a time t0, by means of the TN0 transistor, and then to cut off the TN0 transistor and to open the TP0 transistor for a time t1. The inductance L stores the energy during the period t0 and sends it back during t1, by trying to oppose the variations in the current.
Consequently, regardless of the voltage at the OUT terminal, the inductance L will increase the voltage at the IN terminal so that it can supply the current that the TN0 transistor has demanded during the time t0. This technique therefore allows a voltage Vout to be generated that is greater than the battery voltage Vbat, whilst also supplying current, and therefore acting as a power supply.
Depending on the current Iout demanded by the load, the cyclical relationship t0/(t0+t1) simply needs to be increased. FIG. 1 illustrates two examples of command H, where Iout is low (11) or high (12). The longer the time t0, the greater the current at the end of the t0 period will be, and the more energy will be stored by the inductance L. Consequently, the current supplied by during the period t1 is higher.
To obtain this H command, a REG regulator compares the output voltage Vout with a reference voltage Vref. (for example V ref.=1.6 V) and acts on a pulse width modulator (PWM) which varies the cyclical relationship t0/(t0+t1). If Vout is smaller than the desired voltage, the cyclical relationship is increased. Inversely, if Vout is higher than the desired voltage, then this cyclical relationship is reduced.
A converter of this type is efficient, between 75% and 98% (whereas the efficiency of a switched capacity converter is less than 50%).
Furthermore, it is easier to switch from the pump mode (if Vbat<Vout desired) as mentioned above, to the regulation mode (if Vbat>Vout desired). This latter case may occur if, for example, a radio telephone with a fully charged battery supplies 5.5V when the Vout should be 5V.
FIG. 2 illustrates the operation in regulation mode. The PWM is then stopped as soon as Vbat>Vout desired, and the REG regulator output is connected directly to the TP0 transistor grid. We thus obtain an analogue voltage instead of a digital voltage, linearly regulated.
In order to avoid damaging the components of the circuit powered, in particular if there is a short circuit on the output, or to detect an error, then the Iout current supplied to the load must also be checked. For example, if the latter is a chip card with a maximum consumption of 60 mA, then the power supply must be cut off if the current exceeds a threshold, for example 100 mA. In fact, this means that there is a problem with the card, or that it is inserted incorrectly, or there is an attempted fraud (e.g. a metal plate is inserted). In this case, the system has to be stopped and the output voltage discharged to 0 V.
In order to accomplish this, in the prior art a current/voltage conversion was used as shown in FIG. 3. A small resistor Rext is placed in series with the TP0 (or TN0) transistor to avoid penalising the performances. An amplifier 31 is connected to the two terminals of this resistor Rext. and it delivers the overload signal when the current exceeds the determined threshold.
It is not possible to use a resistor installed onto the silicium, which can vary on a scale of 1 to 3 (thus a risk of error when measuring the current of around 100%). This resistor must therefore be installed outside of the circuit.
This technique has a number of drawbacks, in terms of the cost (small high accuracy resistors are expensive), installation, size, etc. Installing a resistor of less than 1 Ω is not easy. The detection of 100 mA implies that there is at least 100 mV present (100 mA×1 Ω=100 mV) and therefore dissipation of the power of 0.1×0.1=10 mW.
In other terms, this technique is not very accurate: measurement is only carried out during the t1 time (or the t0 time, if the Rext resistor is in series with the TN0 transistor). This measurement is instantaneous and not a mean measurement. In other words, it does not detect the variations in the cyclical relationship.
Finally, with the possibility of switching from the pump mode to the regulation mode, the Rext resistor can only be installed in series with the TP0 PMOS transistor. This means that an extra input/output is required on the circuit for this precise external resistor.